1. Field of the Invention
The present invention is directed toward a method, apparatus and system for determining accurately the coordinates of a sensor (or a transducer including a sensor) relative to a predetermined reference location, particularly where the sensor is part of a transducer for ground penetrating radar.
2. Discussion of Related Art
Ground penetrating radar (“GPR”) systems are used to obtain measurements of subsurface structures and provide images of the internal structure of opaque materials such as soil, rock, concrete, asphalt and wood.
Most GPR equipment utilizes time-domain methods to penetrate a medium. This typically entails generation and radiation of short electromagnetic pulses with center frequencies in a range of about 10 megahertz (MHz) to 2 gigahertz (GHz). The radiated pulses propagate from a system's radar transmitter and transmitting antenna, penetrate the subsurface medium and reflect, refract, and/or diffract at boundaries of intrinsic impedance contrasts, commonly referred to as targets, in the subsurface medium. A portion of the redirected energy propagates back to a receiving antenna (which serves as a sensor), from which the energy may be detected, processed, displayed and stored. In this manner, a time-versus-distance map of a series of measurements over the medium surface can be obtained and used to construct a cross-sectional image of targets within the medium.
A less common GPR technique utilizes stepped continuous-wave technology, which entails the radiation of short pulses at different frequencies. Another rarely used GPR technique employs a continuously radiated pulse swept over a range of frequencies. The data obtained using both these methods can be converted into an equivalent time-distance map as that produced by time-domain GPR systems by using an inverse Fourier transform performed on each scan.
Data is often collected along a series of parallel profile lines to yield a 3-D image of reflectors (targets) in the subsurface medium. The quality of the 3-D image is directly correlated to the accuracy of the coordinates of the parallel profile lines. A conventional method for collecting data along parallel profile lines and demarcating the starting and ending (x,y) coordinates of each profile line is to position the transducer on a grid and move the transducer along each line on the grid. Often, the starting point of each parallel line is indicated by a user-generated mark in the data or by separating the data from each profile line into separate data files. Both methods rely on the user to position the transducer accurately on the starting point of each profile line and to record the (x,y) coordinates of the starting point of each line and the direction of the profile line.
Generally it may be desired that the transducer will traverse along a straight line so that the ending y-coordinate is the same as the starting y-coordinate (assuming travel in the x-direction). However, this is not always the case, particularly if the area being surveyed is large and there is thus a large distance between the beginning and end of a line. A distance encoder, such as a meter wheel, may be used to record the distance the transducer has traveled and the length of a profile line. If the surface of the area being surveyed is uneven, the wheel may (and often will) skip, thus introducing errors into the distance measurement. In applications where centimeter-level accuracy in the position of the transducer is important, such errors in the distance measurement may be unacceptable. Also, it is often undesirable to rely on the operator to position the transducer accurately, and to record these positions since there is a likelihood of operator error, particularly when a very high level of positioning accuracy is required; and in any event, such efforts increase the time involved to successfully collect the data.
There is thus an established need to improve the accuracy and efficiency of registering the (x,y) coordinates of scans of GPR data. Geophysical Survey Systems, Inc. (GSSI) of North Salem, N.H., USA, has previously developed a positioning method that employs the characteristics of a metal strip reflection in GPR data to accurately position one of the coordinates of GPR scans, either the x- or the y-coordinate depending on the orientation of the profile line (U.S. patent application Ser. No. 09/988,570). This method requires that the reflection from the metal strip be recognized in the GPR data, which may pose problems for certain transducer models and antenna orientations relative to the profile line direction.
Accordingly, there is a need for a method for determining the position of a GPR transducer that is not sensitive to GPR transducer characteristics. It may be desirable that such a method be efficient to employ and utilize an apparatus that is inexpensive and simple to deploy and use.